Photographer Daniel Stoupin’s first exposure to the world of marine fluorescence happened in Thailand on a night dive. “What I saw that night was out of this world: The entire reef was glowing in the dark with all possible colors,” he told Weather.com. “No picture can deliver all the emotions that a diver experiences.”

Stoupin’s work comes close, offering a rare look at underwater creatures that literally glow in the dark.

Believe it or not, the concept of fluorescence is pretty common, particularly underwater. Jellyfish may come to mind, but they are actually bioluminescent, not fluorescent, meaning they generate light due to a chemical reaction from within. Coral, shrimp and crabs, on the other hand, light up the black ocean with colors (in the presence of light) by absorbing and emitting light of different wavelengths.

“The underwater world is dramatically different from ours in terms of light absorption and transmission properties,” Stoupin said. “Water absorbs red parts of the spectrum very fast, immersing everything in blue and green.”

Stoupin provides the comparison of grass on land to coral beneath the waves. Grass looks green to our eyes because it reflects green light but absorbs that which is red and blue. Underwater, however, a coral could appear green even if it doesn’t reflect green light; its color comes from its pigments reacting to blue light.

Put another way, fluorescence is subtle, wrote University of Florida biologist Mikhail V. Matz for NOAA’s Ocean Explorer. “It happens when a fraction of the light illuminating an object is absorbed and then re-emitted as a different color.”

OK, so we have a sense of what’s taking place, but how about why? For coral, scientists speculate it’s to provide a more optimal environment for symbiotic microorganisms like algae they need to survive. New research, published in Nature, suggests looking at fluorescence could help us determine coral health and how they’re reacting to cold and heat.

Some jellyfish likely light up to attract prey and as a defense mechanism, notes a paper in Science. For organisms living on the seafloor, the brightness may have no purpose at all, according to Matz.

It all fascinated Stoupin, whose foray into photography came by way of science. (He is working on a Ph.D. in marine biology, but his photos are unrelated to his research.) Some of the macro shots he took in the ocean, others in a studio of material collected from Australia’s Great Barrier Reef. He said ultimately, he’s trying to give these fluorescent organisms their due: “The glowing world deserves far more attention than it gets today.”